BOSTON » Authorities investigating the deadly bombings at the Boston Marathon have recovered a piece of circuit board that they believe was part of one of the explosive devices, and also found the lid of a pressure cooker that apparently was catapulted onto the roof of a nearby building, an official said today.

A law enforcement official briefed on the investigation confirmed to The Associated Press that authorities have recovered what they believe are some of the pieces of the explosive devices. The official spoke on condition of anonymity because this person was not authorized to publicly discuss evidence in the ongoing investigation.

A person close to the investigation previously told AP the bombs consisted of explosives put in 1.6-gallon pressure cookers, one with shards of metal and ball bearings, the other with nails.

Investigators in white jumpsuits fanned out across the streets, rooftops and awnings around the blast site in search of clues today. They combed through debris amid the toppled orange sports drink dispensers, trash cans and sleeves of plastic cups strewn across the street at the marathon’s finish line.

Also today, a doctor at Boston Medical Center said two patients, including a 5-year-old child, remain in critical condition there. Dozens of others have been released from hospitals around Boston.

Law enforcement agencies pleaded Tuesday for the public to come forward with photos, videos or any information that might help them solve the twin bombings that killed three people and wounded more than 170 a day earlier. Investigators circulated information about the bombs, which involved kitchen pressure cookers packed with explosives, nails and other lethal shrapnel — but the FBI said nobody had claimed responsibility.

"Someone knows who did this," Richard DesLauriers, FBI agent in charge in Boston, said at a news conference where he detailed the type of clues a bomber might have left. "Importantly, the person who did this is someone’s friend, neighbor, co-worker or relative."

President Barack Obama branded the attack an act of terrorism. Obama plans to attend an interfaith service Thursday in the victims’ honor in Boston.

Scores of victims of the Boston bombing remained in hospitals, many with grievous injuries. Doctors who treated the wounded corroborated reports that the bombs were packed with shrapnel intended to cause mayhem. In addition to the 5-year-old child, a 9-year-old girl and 10-year-old boy were among 17 victims listed in critical condition.

The trauma surgery chief at Boston Medical Center says most of the injuries his hospital treated after the marathon bombings were to the legs.

"We have a lot of lower extremity injuries, so I think the damage was low to the ground and wasn’t up," Dr. Peter Burke said. "The patients who do have head injuries were blown into things or were hit by fragments that went up."

Dozens of patients have been released from hospitals around the Boston area.

At Massachusetts General Hospital, all four amputations performed there were above the knee, with no hope of saving more of the legs, said Dr. George Velmahos, chief of trauma surgery.

"It wasn’t a hard decision to make," he said Tuesday. "We just completed the ugly job that the bomb did."

An intelligence bulletin issued to law enforcement includes a picture of a mangled pressure cooker and a torn black bag that the FBI said were part of a bomb that exploded during the marathon.

The bombs exploded 10 or more seconds apart, tearing off victims’ limbs and spattering streets with blood. The blasts near the finish line instantly turned the festive race into a hellish scene of confusion, horror and heroics.

The blasts killed 8-year-old Martin Richard, of Boston, and 29-year-old Krystle Campbell, of Medford. The Shenyang Evening News, a state-run Chinese newspaper, identified the third victim as Lu Lingzi. She was a graduate student at Boston University.

Officials found that the bombs in Boston consisted of explosives put in ordinary, 1.6-gallon pressure cookers, one with shards of metal and ball bearings, the other with nails, according to a person close to the investigation who spoke on condition of anonymity because the probe was still going on.

Both bombs were stuffed into black bags and left on the ground, the person said.

Pressure-cooker explosives have been used in international terrorism, and have been recommended for lone-wolf operatives by al-Qaida’s branch in Yemen.

But information on how to make the bombs is readily found online, and U.S. officials said Americans should not rush to judgment in linking the attack to overseas terrorists.

Pressure-cooker explosives have been used in Afghanistan, India, Nepal and Pakistan, according to a July 2010 intelligence report by the FBI and the Homeland Security Department. One of the three devices used in the May 2010 Times Square attempted bombing was a pressure cooker, the report said.

Investigators in the Boston bombing were combing surveillance tapes from businesses around the finish line and asking travelers at Logan Airport to share any photos or video that might help.

"This is probably one of the most photographed areas in the country yesterday," said Boston Police Commissioner Edward Davis. He said two security sweeps of the marathon route had been conducted before the blasts.